The invention concerns an electric motor drive arrangement and in particular a drive for the shaft of a turbo molecular pump operating according to the principle of a brushless d.c. motor.
In this specification, references will be made to the following sources:
1. Siemens Technical Communications from the field of Components PA0 2. Valvo Technical Information for Industry PA0 3. Muller: Two pulse d.c.motors without collectors asr. digest for applied drive technology issue 1/2/1977 PA0 4. Kappius, Liska: Electronic motors for industrial application. Siemens Journal 45 (1971) issue 4. PA0 5 Instructions for use for turbo molecular pumps TPH 110 and drive electronics TCP 270. PA0 No. PM 800 061 BD Firm A. Pfeiffer Vakuumtechnik Wetzlar GmbH, Postfach 1280 D-6334 Asslar. PA0 1. The four-phase winding fills up the winding space poorly, which results in increased ohmic losses. PA0 2. The magnetic energy still provided upon disconnection of the respective winding is not regenerated, but is used up in the Zener diodes or the switching transistors. PA0 3. With defective switching transistors in the operative voltage controller, the full intermediate d.c. voltage Uz occurs at the motor, which leads to the running away of the operative rotational speed and thus to a safety risk for the turbo molecular pump. Moreover with defective current control in the intermediate circuit the switching transistors T.sub.5 to T.sub.8 can be destroyed by overloading. PA0 4. The described examples require either a large mains transformer for the separation of mains voltage and motor voltage or a second control circuit for the stabilisation of the intermediate circuit voltage Uz.
Macek, Hirschmann ia: Switching Power Supplies, IB 1703 PA1 Ackermann et. al: Switching Power Supplies, IIB 1764 PA1 Schaller: Switching Power Supplies, IIIB 1821 PA1 Macek: Switching Power Supplies, IV 1749 PA1 Van Velthooven: Dimensioning of flow transducers PA1 Hedderscheid: No. 76 11 05 Nov. 1976
Brushless d.c. motors are known from publications concerning small type motors (3), (4). They illustrate various alternatives of the phase windings with the necessary switching elements. The described motors have a low power consumption and the driven shafts small moments of inertia, so that the motors quickly reach their operative rotational speed. For these reasons a simple voltage supply is possible.
With turbo molecular pumps the drive motors are located in a vacuum. The rotors of these pumps rotate with a very high frequency (up to 90,000 revs/min). Their operating efficiency depends practically only on the bearing friction. Since the rotors store a large amount of rotational energy on account of their high moment of inertia and the high rotational speed, a very high torque is necessary for the acceleration of the rotor to bring the pump to its operative rotational speed in the shortest time possible. During this starting phase the rotor is accelerated with constant torque, which with d.c. motors requires an operation with constant current. The brushless d.c. motors used for this must accordingly be adapted to these operating conditions.
A brushless d.c. motor with four phase windings for the drive of turbo molecular pumps is known from the company document (5). The current supply of this motor is illustrated in FIG. 2. It consists of a mains transformer with a rectifier for the production of an intermediate d.c. voltage Uz, and a voltage controller which is preferably constructed as a switching controller with a storage coil. During the run-up the current supply so controls the operating voltage U.sub.b that the total current of the motor, which is measured at the current resistor R.sub.1, remains constant. Upon reaching of the operative rotational speed the tachometer takes on the control function and controls the voltage controller so that the operative rotational speed remains constant.
Furthermore, switching controls are known, for example from (1) and (2), which replace the mains transformer (FIG. 3). Within a chopping stage the rectified mains voltage is chopped, with the help of switching transistors having a higher switching frequency compared to the mains frequency, and by means of a transformer an intermediate voltage Uz for the motor is produced. The switching transistors are controlled by means of a pulse width modulator which by means of an automatic control loop keeps constant the voltage provided by the transformer. The current control during run-up as well as the control of the rotational speed during operation result from a second switching controller (storage coil controller, pulse width modulator) as described in the preceding paragraph.
The circuits described above have the following disadvantages: